The present invention relates to an inverter for controlling and driving an electric motor to be driven at variable speeds, and in particular it relates to a control method and apparatus for controlling the inverter by suppressing an overcurrent and overvoltage arising during acceleration or deceleration of the electric motor.
In the variable speed operation of electric motors by means of an inverter which inverts a direct current into an alternating current with a variable voltage and variable frequency, when a time set for adjusting the speed is too short with respect to the magnitude or inertia of a load under an accelerating or decelerating operation, it is likely that an overcurrent will flow through the inverter. In particular, under a decelerating operation, that inertial energy on the load side is regenerated into the dc voltage side to increase the voltage thereof, sometimes resulting in an excessively large voltage. In order to avoid such problems it has been practiced for all drive apparatuses which employ a conventional inverter to adjust its acceleration and deceleration periods before starting actual operation.
On the other hand, under a steady state operation at a constant frequency, there occurs an overcurrent when the magnitude of the load increases sufficiently to cause an overload irrespective of whether the load increase occurs gradually or rapidly.
As a means for protecting the inverter from an overcurrent and overvoltage occurring due to erroneous setting of the acceleration or deceleration times or due to a load change, there is known a method which stops the inverter when its current or voltage exceeds a first limiting value. In addition to this method, the following method has been practiced, which allows a continuous operation of the inverter without stopping it.
Namely, according to this latter method, in an accelerating operation, while the current is above a second current limiting value which is lower than the aforementioned first limiting value, the increasing of the frequency is stopped. Also, in a decelerating operation, while the current is above a given second current limiting value, or while the voltage is above a second voltage limiting value which is below the aforementioned first voltage limiting value, the decreasing of the frequency is stopped. Further, for protection of any overload in a steady operation mode, while the current is above the second current limiting value which is lower than the first limiting value, the frequency is caused to decrease at a decelerating rate, which has been adjusted and preset before starting the actual operation. As an example of control apparatuses of this kind which have implemented a protective system capable of maintaining a continuous operation, as described above, general-purpose inverters are already on the market.
On the other hand, as another protective system, there is one described in the Japanese Patent Application Laid-open No. 3-265480. This is directed to preventing a motor break-down due to a rapid acceleration or deceleration thereof, which is implemented by correcting a reference acceleration such that the motor current maintains a predetermined allowable motor current value whenever the motor is judged to be accelerating or decelerating. As its correction method, it has been suggested to utilize a difference or ratio between a value of the motor current and the allowable value thereof.
As still another protective system, there is one described in Japanese Patent Application Laid-open No. 57-135699, which discloses an inverter controller for driving ac motors, in which, for a loading current which is above the predetermined, that its acceleration rate is lowered when the motor is in a power-running state, and its decelerating rate is lowered when the motor is in regenerating-running state.
Moreover, according to the Japanese Patent Applications Laid-open Nos. 61-207179 and 61-207180 there have been disclosed control methods for a pulse width modulation tape inverter, which methods control the dc voltage to be input to the inverters by means of an ac-dc converter, and in which it is disclosed that in order to control the deceleration period of time, a comparison is made between the dc voltage and the reference voltage value, or an overcurrent is detected.